The present invention relates to memory storage of information, more particularly to materials, methods and apparatuses involving spectral holeburning for providing a medium for such storage.
The concept of using spectral holeburning for memory storage has been known since the discovery of the phenomenon in condensed matter.
xe2x80x9cSpectral holesxe2x80x9d are regions of reduced absorption (enhanced transparency) of the medium at discrete frequencies of the electromagnetic spectrum (visible light inclusive). Such holes can be created by a source of light such as a laser by selectively tuning the laser frequency to a predetermined value and impinging it on the medium of storage. This brings about changes at the atomic or molecular level altering the optical properties of a material (the medium of storage), only at the predetermined frequency. It is noted that spectral holes are not physical holes.
xe2x80x9cSpectral hole-burningxe2x80x9d is the process of creating or burning the spectral hole.
xe2x80x9cPhoton-gated hole-burningxe2x80x9d is a special type of spectral hole-burning that takes place in the presence of two photons, whereas the holes can be read by one photon. It has the advantage that the process of reading the holes does not destroy the holes.
xe2x80x9cPower-gated hole-burningxe2x80x9d is a special type of photon-gated hole-burning where at higher powers the holes can be burned and can be read at a lower power that hardly affects them.
The potential of spectral holeburning based memory for high density storage has been demonstrated by using what is termed xe2x80x9ctransient spectral holeburningxe2x80x9d; however, these systems would be classified as xe2x80x9cvolatilexe2x80x9d storage systems.
xe2x80x9cTransientxe2x80x9d holeburning is significantly distinguishable from xe2x80x9cpersistentxe2x80x9d holeburning. High density memory storage has been demonstrated by transient holeburning in rare earth doped materials. However, in cases of transient holeburning, if the memory is read, a constant refreshing of the memory (i.e., rewriting) is necessary. It is emphasized that xe2x80x9ctransient spectral hole burningxe2x80x9d relates to xe2x80x9cvolatile memory,xe2x80x9d while xe2x80x9cpersistent spectral hole burningxe2x80x9d relates to xe2x80x9cstorage memory.xe2x80x9d
Single photon persistent holeburning has been demonstrated but has had many disadvantages. Perhaps most notable among these disadvantages is the erasure of memory during the reading process.
The above-noted type of spectral holeburning known as xe2x80x9cphoton-gated holeburningxe2x80x9d offers a solution to this problem. According to photon-gated holeburning, the holeburning takes place in the presence of two photons, while reading of these holes requires only one photon. Thus, according to photon-gated holeburning, the process of reading does not destroy the memory.
The multiplication factor by which the spectral holeburning can increase the storage density is determined by the maximum number of spectral holes that can be burned in an electronic transition. However, due to various material limitations, this number has not been large.
Szabo U.S. Pat. No. 3,896,420 issued Jul. 22, 1975, hereby incorporated herein by reference, discloses transient optical/spectral holeburning as a possible mechanism for memory storage.
Castro et al. U.S. Pat. No. 4,101,976 issued Jul. 18, 1978, hereby incorporated herein by reference, disclose a photon-gated holeburning method for creating persistent spectral holes which are not adversely affected by the reading process.
Other pertinent United States patents include the following, each of which is hereby incorporated herein by reference: Yagyu et al. U.S. Pat. No. 5,255,218 issued Oct. 19, 1993; Jefferson et al. U.S. Pat. No. 5,297,076 issued Mar. 22, 1994; Kodama et al. U.S. Pat. No. 5,478,498 issued Dec. 26, 1995; Gimzewski et al U.S. Pat. No. 5,588,886 issued Aug. 20, 1996; Kubota U.S. Pat. No. 5,746,991 issued May 5, 1998.
Other pertinent publications include the following, each of which is hereby incorporated herein by reference: T. Nishimura, E. Yagyu, M. Yoshimura, N. Tsukada and T. Takeyama, SPIE Proceedings on Photochemistry and Photoelectrochemistry of Organic and Inorganic Molecular Thin Films, eds: A. Frank, M. F. Lawrence, S. Ramesesha, C. C. Wamser, Vol. 1436, 31 (1991); H. Lin, T Wang and T. W. Mossberg, Optics Lett. 20 1658 (1995); X. A. Shen, E. Chiang and R. Kachru, Optics Lett. 19, 1246 (1994); W. H. Kim, T. Reinot, J. M. Hayes and G. J. Small, J. Phys. Chem. 99, 7300 (1995); T. Reinot, W. H. Kim, J. M. Hayes and G. J. Small, J. Chem. Phys. 104, 793 (1996); T. Reinot, W. H. Kim, J. M. Hayes and G. J. Small, J. Opt. Soc. Am. B 14, 602 (1997); S. A. Basun, M. Raukas, U. Happek, A. A. Kapalyanskii, J. C. Vial, J. Rennie, W. M. Yen and R. S. Meltzer, Phys. Rev. B 56, 12992 (1997).
In view of the foregoing, it is an object of the present invention to provide a medium for high density storage of information.
It is a further object of the present invention to provide a method for making a medium for high density storage of information.
It is another object of the present invention to provide such a method which uses fast holeburning.
Another object of the present invention is to provide a composition suitable for having holes burned therein in order to produce a medium for high density storage.
A further object of the present invention is to provide such a composition which is suitable for having holes burned rapidly therein.
The present invention is uniquely based on the application of the photon-gated holeburning process to II-VI materials doped with rare earth ions for achieving fast high density optical holeburning. According to the present invention, rare earth doped II-VI compounds are utilized for obtaining fast low power photon-gated high density rewritable memory. Persistent spectral holeburning is effectuated as the mechanism for information storage. Typical inventive practice prescribes a composition which has the following attributes: polycrystallinity; the presence of a host which is one or more narrow bandgap II-VI compounds; and, the presence of a dopant (with which the host is doped) which is one or more rare earth ions existing in at least two different valence states.
The designation xe2x80x9cII-VI compoundxe2x80x9d is conventionally understood to refer to a compound which is the combination of a group (column) two (xe2x80x9cIIxe2x80x9d) element and a group six (xe2x80x9cVIxe2x80x9d) element of the periodic table. Known group xe2x80x9cIIAxe2x80x9d elements are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra). Known group xe2x80x9cIIBxe2x80x9d elements are zinc (Zn), cadmium (Cd) and mercury (Hg). Known group xe2x80x9cVIAxe2x80x9d elements are oxygen (O), sulfur (S), selenium (Se), tellurium (Te) and polonium (Po). With regard to the present invention, experimentation has primarily involved the II-VI compounds MgS, CaS, BaS and SrS. Beside the II-VI sulfides (II-VI compounds including the group six element sulfur), other II-VI compounds such as II-VI selenides (II-VI compounds including the group six element selenium) appear to be promising and are being investigated.
The known xe2x80x9crare earth elements,xe2x80x9d listed as follows, are conventionally understood to be those sixteen elements having an atomic number of 39 or between 57 and 71 inclusive: yttrium (Y, 39); lanthanum (La, atomic number 57); cerium (Ce, atomic number 58); praseodymium (Pr, atomic number 59); neodymium (Nd, atomic number 60); promethium (Pm, atomic number 61); samarium (Sm, atomic number 62); europium (Eu, atomic, number 63); gadolinium (Gd, atomic number 64); terbium (Tb, atomic number 65); dysprosium (Dy, atomic number 66); holmium (Ho, atomic number 67); erbium (Er, atomic number 68); thulium (Tm, atomic number 69); ytterbium (Yb, atomic number 70); lutetium (Lu, atomic number 71). With regard to the present invention, experimentation has primarily involved the rare earth element europium (Eu); other rare elements have also been tried, notably cerium (Ce), neodymium (Nd), samarium (Sm) and terbium (Tb).
According to many embodiments, the present invention provides a composition which is characterized by polycrystallinity and which comprises at least one II-VI compound and at least one rare earth element with which the at least one II-VI compound is doped. Typically, the at least one rare earth element includes at least two different valence states. Normally, the inventive composition is suitable for having a plurality of persistent spectral holes burned therein in order to produce a medium for high density storage.
The inventors have also considered and are investigating the possibility of inventively implementing transition metal ions other than rare earth ions. That is, an inventive composition (typically, characterized by polycrystallinity) can comprise at least one II-VI compound and at least one rare earth element with which the at least one II-VI compound is doped. Or, an inventive composition (typically, characterized by polycrystallinity) can comprise at least one II-VI compound and at least one transition metal element (instead of at least one rare earth element) with which the at least one II-VI compound is doped. Or, an inventive composition (typically, characterized by polycrystallinity) can comprise at least one II-VI compound and the combination of at least one rare earth element and at least one transition metal element, with which the at least one II-VI compound is doped; typical such embodiments would provide different valence states for the at least one transition metal element and the at least one rare earth element, e.g., a first valence state for a rare earth element and a second (different) valence state for a transition metal element.
In accordance with the present invention, variations of II-VI compounds and/or of the rare earth ions (and/or of the transition metal ions) can be used for covering different wavelength regions of the spectrum and/or for obtaining more promising characteristics pertaining to the phenomenon of holeburning and/or its applications.
Typically, according to this invention, the composition comprises two valence-valued rare earth atoms, viz., (i) a plurality of rare earth ions of a first valence state and (ii) a plurality of rare earth ions of a second valence state which differs from the first valence state. The II-VI compound/compounds is/are doped with (i) the rare earth ions of the first valence state and (ii) the rare earth ions of the second valence state. The rare earth ions are present so as to describe a ratio of the total rare earth ions of the first valence state to the total rare earth ions of the second valence state. According to typical inventive practice involving two different valence-valued rare earth atoms, the atoms of the higher-valued valence state are more numerous than are the atoms of the lower-valued valence state.
The present invention also provides a medium for information storage. The medium typically comprises a polycrystalline composition which includes at least one II-VI compound and at least one rare earth element with which the II-VI compound is doped. Typically, the rare earth element or elements is or are characterized by two different valence states. The composition has, burned therein, a plurality of persistent spectral holes.
The present invention also provides a method of making a medium suitable for information storage. The method of making such a medium typically comprises: providing a polycrystalline composition which includes at least one II-VI compound and at least one rare earth element with which said II-VI compound is doped; and, burning a plurality of persistent spectral holes in said composition. According to typical inventive practice, the rare earth element or elements is or are characterized by two different valence states, and the persistent spectral hole burning is or includes photon-gated persistent spectral holeburning.
Also provided by the present invention is a method of using a II-VI compound. The method of using a II-VI compound typically comprises: doping the II-VI compound with at least one rare earth element so as to form a polycrystalline composition, wherein the at least one rare earth element includes at least two different valence states; and burning a plurality of persistent spectral holes in the composition (typically, wherein photon-gated persistent spectral holeburning is effectuated).
The favorable characteristics of holeburning in accordance with the present invention are believed to be at least largely attributable to three inventive features, viz., (i) the polycrystallinity of the composition; (ii) the presence of two valence states of the rare earth atoms in the host; and, (iii) the suitable band gap of the II-VI host.
Generally according to this invention the composition is polycrystalline rather than single-crystalline in nature. Single crystals are very unlikely to demonstrate either the high density or the fast speed which are associated with polycrystals. Polycrystalline powders are capable of having the right concentration of electron traps, particularly the deep traps that make possible the high-density persistence and high speed of holeburning.
According to the present invention, the host can comprise one II-VI compound (e.g., CaS or MgS), or can comprise two or more II-VI compounds (e.g., CaS and MgS) which are mixed, stratified or otherwise combined. The host can be doped with one or more rare earth elements having one or more different valence states. For instance, MgS can be doped with a single rare earth element (e.g., Eu), or can be xe2x80x9cco-dopedxe2x80x9d with two rare earth elements (e.g., Eu and Sm).
In typical inventive practice, the host is doped with rare earth atoms having two different valence states. The two different valence states can correspond to the same rare earth element (e.g., Eu2+ and Eu3+), or can correspond to two or more different rare earth elements (e.g., Eu2+ and Sm3+; or, Eu2+ and Sm2+ and Sm3+; or, Eu3+ and Sm2+ and Sm3+; or, Eu2+ and Eu3+ and Sm2+; or, Eu2+ and Eu3+ and Sm3+; or, Eu2+ and Eu3+ and Sm2+ and Sm3+).
The high density of the burned holes depends on the ratio of the respective numbers (totals or sums) of rare earth atoms, that is, in terms of how many rare earth atoms have the higher-numbered valence state versus how many rare earth atoms have the lower-numbered valence state. The speed of the holeburning also depends on the ratio of the presence of these two valence states. A xe2x80x9crule of thumbxe2x80x9d for inventive practice, generally, is that the greater the ratio of the respective amounts of the two valence states, the greater the efficiency of the holeburning. In other words, the higher the ratio of these quantities, the better.
According to frequent inventive practice, the ratio of the number of atoms having the higher-numbered valence state to the number of atoms having the lower-numbered valence state will be greater than or equal to approximately 2 and less than or equal to approximately 10 (in other words, fall within the range between about two-to-one and ten-to-one). Ratios higher than ten or lower than two are possible in inventive practice. In general, a ratio of 10 is preferable to a ratio of 9, for instance, and a ratio of 11 is preferable to a ratio of 10.
The inventive medium can include one or more rare earth elements. Selection of the rare earth element or elements will typically be such as to accommodate the right ratio of the two valence states. The medium can comprise any II-VI compound or compounds, wherein the energies are adjusted in the suitable range of the spectrum.
In the case of Eu, for instance, according to typical inventive practice these two valence states are Eu2+ and Eu3+. The ratio of Eu3+ to Eu2+ in terms of prevalence of each is important. In inventive testing this ratio was increased to approximately ten or more (i.e., at least ten to one of Eu3+ to Eu2+). Since polycrystals were used, it was preferable for Eu3+ to be the dominant valence state. If single crystals were used instead of polycrystals, it would have been preferable to allow Eu2+ to be the dominant valance state. It is noted that, generally speaking, II-VI host materials are propitious insofar as allowing the 2+ valence state of the rare earth ion to be substituted therein.
The narrow band gaps of II-VI materials are particularly appropriate for the photoionization photon-gated holeburning made possible by red or IR lasers. This is a region of interest for commercial reasons; compact semiconductor lasers exist for communication purposes in the region of infra-red and approaching the red.
The present invention addresses certain key issues for the application of spectral holeburning technology, including the following: (i) the identification of the materials with high density of spectral holes; (ii) the demonstration of survivability of these holes over reading cycles; (iii) the demonstration of the stability of these holes under thermal cycling and at elevated temperatures; and, (iv) the demonstration of fast burning of holes using nano-second laser pulses.
Spectral holeburning has been known to have the potential for increasing the storage density by several orders of magnitude. Overcome by the present invention are some longstanding hurdles associated with persistent spectral holeburning. This invention combines stability and robustness of the memory with its high density of storage.
This invention is believed to demonstrate the highest number, to date, of persistent holes burned in any materials used. The inventors have burned two hundred forty (240) persistent photon-gated spectral holes in the zero phonon line (ZPL) of the 4f-5d transition of Eu2+ in magnesium sulfide host. This is believed to represent the largest number of persistent photon-gated holes ever burned in any system. In principle, up to about a thousand (1,000) persistent photon-gated spectral holes or more can be burned in accordance with the present invention.
Among other advantages of the present invention, reading the inventively generated holes (memory) requires minimal power (microwatts or even nanowatts).
Another inventive advantage is fast writing and reading of the holes (data). Inventively generated holes were burned by a 10 nanosecond pulse. By comparison, typical burning times in rare earth for gated hole burning are in tens to hundreds of seconds.
Furthermore, the inventively generated holes are persistent and do not suffer any degradation after thousands of reading cycles.
Moreover, thermal cycling up to a temperature of 170 degrees K has little effect on the inventively generated holes.
In addition, the inventively generated holes are quickly erasable (typically, in a range between a few tenths of seconds and hundredths of seconds) by slightly shifting the energy of the laser beam. This photo-erasability has favorable implications for multiple-write memory.
In contrast to the systems according to the present invention, current transient holeburning systems work at or close to 4.2 degrees K. Generally in accordance with these known transient holeburning systems, the memory is lost by any surge in temperature around 10 degrees K or higher; the memory is temporary. The memory is erased eventually within the course of (at most) a day, even at 2 degrees K and when the system is unused. The memory is erased in the process of reading it.
The photon-gated systems of the past afforded various benefits. They were long-lived, and the memory in them was immune to reading. However, the low density of the holes and long burning times rendered these photon-gated systems of little commercial significance. In most cases, a rewriting requires heating the sample from the cryogenic temperatures up to the room temperature in order to erase the memory quickly.
Among the potential commercial uses of the present invention are the following: high density optical memory; time domain; frequency domain; hybrids of time and frequency domain storage; page oriented spectral holographic memory; quantum computing; spectral holographic storage; petaflop computing.
It is believed that the present invention, which provides persistent holeburning materials and systems pertaining thereto which are characterized by high densities of holes, has potential commercial value. Other persistent holeburning materials and systems pertaining thereto are characterized by low densities of holes; if and to the extent that they have been explored, they are believed in general to lack real commercial value.
Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.